Add general introduction for national pages

[LianeHughes]

This is modified from the text originally read for Sweden, general agreement from the national resources teams that this would be a good way to introduce the national pages for now.

[rabuono]

Might be interesting to add a bit more on the aims of having these pages. For example, of signposting resources and initiatives so that other countries can better exchange on the design and implementation of solutions.

[LianeHughes]

Tried to change to make clearer the aim of the page as suggested (very quickly)